The present invention relates to a metal-to-metal seal assembly and, more particularly, to such an assembly for use in off and gas production apparatus located in a well.
It is critically important to properly seal certain components in oil and gas wells during the operation of downhole tools, after completion and testing of the well, and during production. For example, expansion joints, often referred to as "polished bore receptacles," have evolved which are connected in the production tubing string in a completed well to compensate for changes in the axial length of the tubing string due to the effects of relatively large temperature changes in the well, which would otherwise cause a compression deformation or tensile failure. A typical polished bore receptacle includes two tubular members disposed in a telescoping relationship that move relatively to each other in an axial direction in response to temperature variations, and a continuous dynamic seal is provided between the two members to prevent fluid leakage between the sliding surfaces of the two members.
Elastomer seals have been used in a variety of sealing applications in oil and gas wells, including use in the polished bore receptacles described above. However, the elastomer may lose its resiliency or shape memory after some use, which is necessary for the seal to oppose the imposed forces thereon. Also, elastomer seals tend to deteriorate with exposure to the downhole chemical and relative high temperature environments for long periods of time. Further, significant abrasion of the seal material will occur by the forces generated when there is relative movement between the two members being sealed, as is the case with polished bore receptacles. Although these deficiencies can be compensated for to a certain degree by preloading the seal, the preloading force becomes less as more and more of the seal material abrades, ultimately causing seal leakage and failure.
Therefore, to overcome these problems, metal-to-metal seals have evolved since they, for the most part, do not lose their resiliency and shape memory and are not affected by hostile environments. However, metal-to-metal seals are normally only used as static seals or as safety backup seals since the seal must remain stationary and must be under constant compression to insure that it is not compromised. Therefore, these metal-to-metal seals are not suitable for use in dynamic sealing applications, including the polished bore receptacles described above. Therefore, what is needed is a metal-to-metal seal assembly which can seal between two members which move relative to each other and which can maintain the seal without the need for constant compressive loading.